Our preliminary sero-epidemiologic survey of ethnic groups in Israel has shown that 37% of healthy Falasha individuals ("Black Jews" of Ethopian decent) have antibodies for HTLV-I and -II.* This is the highest known prevalence of antibody to HTLV in any population and indicates widespread infection with HTLV. HTLV is an exogenous virus and is associated with the development of several human malignancies. Sero-epidemiologic studies will be performed to (i) characterize the clinically relevant biologic activities; (ii) detect the exact mode of horizontal transmission of the virus; (iii) study virus-host interactions including determinations of host-range and localization and overlap of different types of HTLV. The hypothesis will be tested that prevalence of HTLV antibodies is correlated with antibodies to other viruses implicated in human malignancy, specifically CMV, EBV, and HBV. A cross-sectional study will be done to confirm the hypothesis that HTLV antibodies are associated with specific T-cell malignancy, and to probe for new and unrecognized disease associations. Prospective studies will test the mode of HTLV transmission of mother to child and will characterize the immune response of neonatal HTLV infection. Absorption enzyme immunoassay studies suggest that sero-positive Falashas carry a new subtype of HTLV that crossreacts with HTLV types I and II but differs from them. HTLV will be isolated from the lymphocytes of seropositive Falasha individuals, grown in culture, and purified in quantity. An ELISA test will be established and competition immunoassays among HTLV types -I, -II, and -Falasha will be performed. HTLV isolated from Falasha individuals will be molelcularly cloned, restriction maps will be derived, and selected regions of the viral genome will be sequenced and compared with other subtypes of the group. * In addition, 10% of these have antibodies to HTLV-III as well.